python exemplary

Python Exemplary

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CAMERA

The source code of all examples can be downloaded from here.

Take a snapshot and store image locally

Small and cheap photo and movie cameras can easily be attached to the Raspberry Pi's camera port. There are many good tutorials on the Internet that explain how to install the software and program the camera using the excellent picamera Python module. (The software is preinstalled in our SD-card distribution.) We modularize the code even more by defining a few well-documented functions in the file camera.py that you get from here and that must reside in the same folder as your programs.

Program:[►]

# camera.py

import picamera
import StringIO

def captureJPEG(width, height):
    '''
    Takes a camera picture with given picture size and returns the image
    in JPEG format. The picture resolution is width x height (max: 5 MPix)
    @param width: the width of the picture in pixels (max: 2592)
    @param height: the height of the picture in pixels (max: 1944)
    return: the image in JPEG format (as string); None, if the capture fails
    '''
    camera = picamera.PiCamera()
    imageData = StringIO.StringIO()
    w = int(width)
    h = int(height)

    try:
        camera.capture(imageData, format = "jpeg", resize = (w, h))
        imageData.seek(0)
        data = imageData.getvalue()
        return data
    finally:
        camera.close()
    return None  # error

def saveData(data, filename):
    '''
    Writes the given string data into a binary file.
    @param data: the data to store (as string type)
    @param filename: a valid filename in the local file space
    '''
    file = open(filename, "wb")
    file.write(data)
    file.close()

def captureAndSave(width, height, filename):
    '''
    Takes a camera picture with given picture size and stores is
    in JPEG format.
    The picture resolution is width x height (max: 5 MPix)
    @param width: the width of the picture in pixels (max: 2592)
    @param height: the height of the picture in pixels (max: 1944)
    @param filename: a valid filename in the local file space, 
    e.g. /home/pi/shot1.jpg
    '''
    data = captureJPEG(width, height)
    if data != None:
        saveData(data, filename)

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Remarks:
To avoid to create a temporary disk file, captureJPEG() holds the image data in a "memory file" using StringIO.

Aim:
Attach a camera, take a snapshot and store the JPEG file on the local file system.

Program:[►]

# Camera1.py

import camera

print "Capturing image..."
img = camera.captureJPEG(300, 200)
print "size", len(img)
camera.saveData(img, "/home/pi/test.jpg")
print "JPEG saved"

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Remarks:
To check the result, you have several choices: You can start a VNC session and display test.jpg by a double-click in a Raspberry Pi JPEG viewer. You may also upload the file to your PC and display the image there using any picture viewer.

There are many nice projects using the camera, among them a surveillance system that detects if the image changed due to a person that enters the scene.

Take a snapshot and transfer image to a remote system

We seize the opportunity to demonstrate how to transfer data from the Raspberry Pi to a remote client by using a TCP client/server system. The code is not explained in detail, but with the help additional information found on the internet, you should be able to fully understand it.

Aim:
Implement a socket server on the Raspberry Pi that may be accessed via TCP using a user-defined IP port (here port 22000). Write a Python client program that runs on a remote system. After connection, the client sends the command "go" to the server to trigger a camera snapshot. The JPEG is then transferred without any user action from the Raspberry Pi to the client and displayed there.

The main program starts a server listening loop, where the blocking function serverSocket.accept() waits for incoming client connection requests. After connection, server starts a new thread SocketHandler, where it handles the communication with this particular client. This is not really necessary in this situation, because there is only one client to serve, but we opt for this standard program design that is used in many other client-sever applications.

Program:[►]

# CameraServer.py

from threading import Thread
import socket
import time
import camera

VERBOSE = False
IP_PORT = 22000

def debug(text):
    if VERBOSE:
        print "Debug:---", text

# ---------------------- class SocketHandler ------------------------
class SocketHandler(Thread):
    def __init__(self, conn):
        Thread.__init__(self)
        self.conn = conn

    def run(self):
        global isConnected
        debug("SocketHandler started")
        isRunning = True
        while isRunning:
            cmd = ""
            try:
                debug("Calling blocking conn.recv()")
                cmd = self.conn.recv(1024)
            except:
                debug("exception in conn.recv()") 
                # happens when connection is reset from the peer
                break
            debug("Received cmd: " + cmd + " len: " + str(len(cmd)))
            if len(cmd) == 0:
                break
            rc = self.executeCommand(cmd)
            if not rc:
                isRunning = False
        conn.close()
        print "Client disconnected. Waiting for next client..."
        isConnected = False
        debug("SocketHandler terminated")

    def executeCommand(self, cmd):
        debug("Calling executeCommand() with  cmd: " + cmd)
        if cmd == "go":
            print "Taking snapshot and transfer to client..."
            jpg = camera.captureJPEG(640, 480)
            return self.sendData(jpg)
        if cmd == "disconnect":
            debug("Client request disconnection")
            return False

    def sendData(self, data):
        debug("sendData() with len: " + str(len(data)))
        try:
            self.conn.sendall(data)
        except:
            return False
        return True
# ----------------- End of SocketHandler -----------------------

serverSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# close port when process exits:
serverSocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) 
debug("Socket created")
HOSTNAME = "" # Symbolic name meaning all available interfaces
try:
    serverSocket.bind((HOSTNAME, IP_PORT))
except socket.error as msg:
    print "Bind failed", msg[0], msg[1]
    sys.exit()
serverSocket.listen(10)

print "Waiting for a connecting client..."
isConnected = False
while True:
    debug("Calling blocking accept()...")
    conn, addr = serverSocket.accept()
    print "Connected with client at " + addr[0]
    isConnected = True
    socketHandler = SocketHandler(conn)
    # necessary to terminate it at program termination:
    socketHandler.setDaemon(True)  
    socketHandler.start()
    t = 0
    while isConnected:
        print "Camera ready at", t, "s"
        time.sleep(10)
        t += 10

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Remarks:
Consult another tutorial about socket programming to fully understand the code. We pay particular attention to a clean termination of the handler thread and wait for the next client connection.

The client code also creates a user-defined thread Receiver that handles the incoming data independent of the main program. Again this is a usual scenario for client-server applications. Data is read in chunks with maximal 4096 bytes until an end-of-file signature is found. For JPEG files the last two bytes are 0xFF, 0xD9.

The JPEG image is then displayed in a TigerJython GPanel graphics window. Closing the window by clicking the title bar close button disconnects the client and puts the server into a listening state for the next client connection.

Program:[►]

# CameraClient1.py
# Display image in TigerJython's GPanel graphics window

from threading import Thread
import socket, time
from gpanel import *

VERBOSE = False
IP_ADDRESS = "192.168.0.12"
IP_PORT = 22000

def debug(text):
    if VERBOSE:
        print "Debug:---", text

# --------------------- class Receiver ---------------------------
class Receiver(Thread):
    def run(self):
        debug("Receiver thread started")
        while True:
            try:
                rxData = self.readServerData()
            except:    
                debug("Exception in Receiver.run()")
                isReceiverRunning = False
                closeConnection()
                break
        debug("Receiver thread terminated")

    def readServerData(self):
        global isJPEG
        debug("Calling readResponse")
        bufSize = 4096
        data = ""
        while data[-2:] != "\xff\xd9":
        # eof tag for jpeg files (both chars must be in same block)
        # We are not sure 100% that this sequence is never embedded in image
        # but it is improbable to happen at the end of the data block
            try:
                blk = sock.recv(bufSize)
                if blk != None:
                    debug("Received data block, len: " + str(len(blk)))
                else:
                    debug("sock.recv() returned with None")
            except:
                raise Exception("Exception from blocking sock.recv()")
            data += blk
            print "JPEG received. Displaying it..."
            display(data)
# -------------------- End of Receiver ---------------------

def startReceiver():
    debug("Starting Receiver thread")
    receiver = Receiver()
    receiver.start()

def sendCommand(cmd):
    debug("sendCommand() with cmd = " + cmd)
    try:
        sock.sendall(cmd)
    except:
        debug("Exception in sendCommand()")
        closeConnection()

def closeConnection():
    debug("Closing socket")
    sock.close()

def connect():
    global sock
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    debug("Connecting...")
    try:
        sock.connect((IP_ADDRESS, IP_PORT))
    except:
        debug("Connection failed.")
        return False
    startReceiver()
    return True
 
def display(data):
    img = readImage(data)
    if img != None:
        image(img, 0, 0)   

def onExit():
    global isRunning
    isRunning = False
    dispose()

width = 640
height = 480
makeGPanel(Size(width, height))  
addExitListener(onExit)
sock = None
isRunning = True   

if connect():
    print "Connection established"
    time.sleep(1)
    while isRunning:
        print "Sending command 'go'..."
        sendCommand("go")
        time.sleep(2)
    print "Disconnecting now..."    
#    sendCommand("disconnect")    
    closeConnection()
else:
    print "Connection to %s:%d failed" %(IP_ADDRESS, IP_PORT)
print "done"

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Remarks:
If you don't have TigerJython, you may also copy the incoming image to a disk file and display it with a standard image viewer. Just add the function saveData() and modify display().

Program:[►]

# CameraClient2.py

import webbrowser

def saveData(data, filename):
    file = open(filename, "wb")
    file.write(data)
    file.close()

def display(data):
    jpgFile = "c:/scratch/test.jpg"
    saveData(data, jpgFile)
    webbrowser.open(jpgFile)

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